This project directly addresses the idea of etiological heterogeneity in ADHD in a prospective longitudinal design. Clarification of exophenotypic factor structure, in particular the role of sluggish cognitive tempo and of different assessment methods, is viewed as essential to provide an interpretive context for findings on mechanism heterogeneity. The first aim therefore utilizes about 13,000 teacher ratings of symptoms of ADHD, oppositional/conduct, and sluggish tempo items to evaluate the fit of two, three, and four factor models of the domain. The second aim instantiates a multi-level, multiple-pathway model in which the core constructs assessed are [a] regulatory control (related to the idea of cognitive control, executive function, and prefrontal neural systems) and [b] reactive response or approach (related to the idea of reward delay discounting and reinforcement learning). These constructs are assessed at three levels: cognitive task performance, physiological response, and child and parent trait ratings. Inclusion of multiple measures of both domains in the same study is among its chief innovations. Important moderating mechanisms will also be assessed, namely arousal/alertness and temporal information processing, and key family context variables are also assessed as moderators. 500 children will be recruited at age 7 years via a two stage population based sampling method in an effort to maximize the generalizability of the heterogeneity models that best fit these data. Analyses will evaluate which mechanism based measure best relates to different symptom domains and different ADHD subtypes. The third aim more ambitiously attempts to break new ground in identifying ADHD subgroups validated by mechanism differences, using typological approaches to both the large teacher-rated sample and the 500 children with multiple measures of the core constructs. Three waves of data will be collected annually, and longitudinal analyses under each aim will evaluate types not only cross sectionally, but in terms of stability and change over time. It is hypothesized that when ADHD is accompanied by neurocognitive weakness it will have a more stable or deteriorating course. Results are aimed at making significant advances in empirical description of causal heterogeneity in ADHD.